Electron beam lithography uses focused electron beams to expose electron-sensitive resists on wafers. There are two main methods of electron emission - thermionic emission from a heated tungsten tip or field emission from a sharp tungsten wire. Raster and vector scanning are used to direct the electron beam during exposure. Positive and negative tone electron beam resists like PMMA and SAL601 are commonly used. Ion beam lithography is an alternative technique that uses focused ion beams, usually of hydrogen, helium or argon, which interact chemically with resists for high sensitivity and minimal scattering.

1. Electron Beam Lithography

2. Electron Beam Lithography
Schottky emission (SE) and cold field emission (CFE) have been in common use, especially for
nanometer-sized beams for electron focusing systems. Emission of electrons from a metal under the influence of a field
occurs in both SE and CFE.
During SE, a blunt tungsten emitter tip coated with a low work function material (ZrO) is heated to 1800 K, and
thermionic emission takes place; that is, heat thermally excites the electrons enough to bring them out of the material.
During CFE, a much smaller tungsten wire (radius of <0.1 μm) is used, and a very high field causes electrons to tunnel
out of the material. In CFE sources, electrons tunnel from various energies below the Fermi level.

3. Electron Beam Lithography
There are two basic ways to scan an electron beam. In raster scanning, the patterns are written by an electron beam
that moves through a regular pattern. The beam scans sequentially over the entire area and is blanked off where no
exposure is required.
On the contrary, in vector scanning, the electron beam is directed only to the requested pattern features and hops
from features to features. Therefore, time is saved in a vector scan system.

4. Electron Beam Lithography
Electron-Beam Resists:
1. PMMA – Positive electron beam resist
2. Microposit SAL601 (Shipley) - Negative electron beam resist
3. A copolymer of glycidyl methacrylate and ethyl acrylate (COP) - Negative electron beam resist
Applications: Used in manufacturing of Masks for photolithography
Source materials:

5. Electron Beam Lithography- SCALPEL
Scattering with angular limitation projection electron-beam lithography
• The mask is a continuous thin membrane (∼100 nm)
made of a low atomic number material like silicon
nitride, patterned with a 25–50-nm-thick layer of a
high atomic material such as tungsten.
• High-energy electrons pass through both the silicon
nitride and the tungsten, but the tungsten scatters
the electrons widely, whereas the silicon nitride
results in very little scattering. An aperture blocks
the scattered electrons, resulting in a high-quality
image at the wafer. Aperture also absorbs unwanted
energy thereby minimizing the thermal instabilities
of the the mask .

6. Deep UV Lithography
• Wavelength of radiation: 254–193-nm
• Mask: chrome-on-quartz photomask
• Optical reduction through lens which reduces the pattern size by a factor of 5 or 4 times
• CAD data format: The pattern data is converted into a standardized file format, usually GDSII or, more
recently, OASIS.
• Coating of photoresist: Spin coating as in photolithography, a Bottom Antireflective Coating (BARC) is
usually applied to the surface of the wafer by spin-coating, BARC absorbs small wavelength light without
transferring to wafer. Sometimes TARC is also applied above resist surface.

7. Deep UV Lithography
• The brightness of shorter wavelength sources is severely reduced compared with that of longer wavelength
sources, and the addition of lenses further reduces the efficiency of the exposure system. As a consequence, with
shorter wavelengths, higher resist sensitivity is required, and newer deep UV sources that produce a higher flux
of deep UV radiation must be used.
• Photoresist advancements: CA ( Chemically amplified resists) more sensitive to DUV. They produce increased
resolutions.
• Source: KrF excimer laser with a short wavelength of 248 nm and a power of 10 - 20 W at that wavelength is
used.
• Recent Sources: ArF excimer lasers (193 nm), F2 Lasers (157 nm)
• Wafer Priming: typical resist adhesion promoting primer is 1,1,1,3,3,3-hexamethyldisilazane (abbreviated
HMDS) [formula: (CH3)3SiNHSi (CH3)3], developed and patented by IBM.

8. Ion Beam Lithography
• Source Beam: uniform beam of hydrogen or helium ions at
about 10 kV. Also Ar+
• E x B mass separation unit
• Stencil mask: Si membrane with holes. The mask fabrication
process involves silicon-on-insulator (SOI) and dry etching.
• Demagnified 4x on the mask
• Scattering is very less compared to all other techniques
• Resist: The resist can be an ordinary PMMA resist that absorbs
most of the ions during exposure
• The most important application of focused ion beams remains
the repair of masks for optical or x-ray lithography, a task for
which commercial systems are available.
• Chemically reactive with surface.
• When operated at low beam currents, the system can be used
for imaging, and when operated at high-beam currents, it can
be used for site-specific sputtering or milling.
• Achievable 32 nm size feature.

9. Ion Beam Lithography
• Ion-beam lithography has at least two advantages over EBL:
• 1) It has almost two orders of magnitude higher resist sensitivity, and
• 2) It has negligible ion scattering in the resist and very low backscattering from the substrate.
• A major problem is the potential for damage to sensitive electronic functions from the high energy ions.